Digital color printers print images based upon digital image data. This digital image data includes code values indicating the colors to be printed in an image. When a digital color printer generates a printed image, it is intended that the printed image will contain the exact colors called for by the code values.
However, in practice, it has been found that the colors in the images printed by digital color printers do not always match the colors called for by the code values. Such color variations can be caused by differences in the composition or fabrication of donor materials such as inks, thermal donor, or toner, by differences in the composition of receiver materials, such as paper, fabrics, film, or other substrates, by variations in the assembly or design of the printer, and/or by day-to-day or even hour-to-hour variations in printer operations, such as might be caused by changes in environmental conditions.
What is needed therefore is a printer that is capable of self-calibration. One such printer is described in U.S. Pat. No. 6,086,274 in which a so called line printer is described having a color measuring head for color reflection measurements. The color measuring head is arranged to be fixed to a printer carriage in lieu of or in addition to the printing head, with the color measuring head being oriented toward a paper passage plane. The color measuring head is in communication with amplifying, control and evaluating electronic units arranged internally or externally of the printer. The color measuring head enables the line printer to be used not only for its inherent printing function but also for colorimetric measurements and for computing and using a color profile, to thereby automate a color characterizing process. In the '274 patent, the measuring head can automatically scan, line by line, the color fields of a previously printed test chart and the printing mechanism can be utilized for continuous scanning operation. Using the arrangement described in the '274 patent, the printing and measuring functions are performed in one run of a test chart. The printing and measuring operations are offset by a small time interval only, which is especially advantageous if the color measurement is not effected on a test chart but rather during the output of images. Measurements of this latter type are described as being performed directly in the image area of interest if a predetermined color is required to be maintained at predetermined portions of a printed image.
However, it will be appreciated that the printing of a test chart for calibration delays printing of images and that consumes meaningful amounts of receiver media that could otherwise be used for printing images. Conversely, not all of the colors required for calibration will necessarily be printed in images that are printed within a time period within which it may be necessary to complete calibration or printed within areas that are sized to permit color measurement.
Accordingly, what is needed is a self-calibrating printer that is adapted to execute an on demand self-calibration processes without requiring the printing of separate test images, using little if any additional receiver media, and without delaying the printing of images.